The present invention relates generally to the art of plasma cutting. More specifically, it relates to plasma cutters having a gas supply for the cutting torch.
Plasma arc cutting, or plasma cutting, is a process in which an electric arc is used to cut a metallic workpiece. Generally, plasma arc cutting uses an electric arc between an electrode and the metal to be cut. The arc creates a plasma that cuts the metallic workpiece. The electrode is part of a cutting torch.
There are a number of known methods of initiating a plasma arc discharge and starting an arc plasma torch (for plasma cutting). Often, a pilot arc is drawn between a cathode and an anode, and an ionizable gas is directed to flow around the arc, creating a plasma jet. The arc is then re-directed to the workpiece.
One common method for starting the pilot arc, called contact starting, uses a specially designed torch and nozzle. An example of a contact start torch is described in U.S. Pat. No. 4,791,268, to N. Sanders, et al., and U.S. Pat. No. 4,902,871, to N. Sanders, et al., hereby incorporated by reference. When an operator triggers the contact start torch, the pilot circuit generates a pilot current between a touching electrode and torch nozzle. Also, a valve in the torch is opened, and an ionizable gas flows through the torch. The gas moves the electrode away from the nozzle, creating a pilot arc therebetween. The plasma torch then is brought near the workpiece to initiate the cutting arc, and the gas flowing through the torch causes the arc to transfer to the workpiece.
Another prior art plasma cutting system and torch is described in Method And Apparatus For A Contact Start Plasma Cutting Process, issued Aug. 26, 1997 as U.S. Pat. No. 5,660,745, and is hereby incorporated by reference.
Generally, prior art plasma systems (also called plasma cutters or plasma cutting power supplies) include a source of gas (such as a tank, shop air, or a compressor), a power supply, a controller, and a torch. The power source typically includes the valve that regulates the flow of gas. A valve may also be included in the torch, or elsewhere between the torch and the source of air.
The system may include a pressure sensor to sense the pressure of the gas supply. If the pressure is low, the controller disables the systemxe2x80x94when the trigger is pulled the pilot current will not be provided, and the gas valve in the torch is not opened. An indicator, such as an LED, may be provided to notify the user that an inadequate gas supply caused the system not to initiate when the trigger was pulled. This system works fine when the gas supply is so inadequate that the pressure is too low even when no gas is flowing.
Unfortunately, such prior art system have a drawback. It is possible for the gas supply to be marginally inadequatexe2x80x94provide sufficient pressure when the valve is closed (and no gas is flowing), but provide insufficient pressure when the valve in the torch is opened. Thus, before starting the pressure is adequate. The user pulls the trigger, and when the valve opens the pressure drops (below a desired threshold). This disables this system, and lights the low pressure indicator, or damages components. But, the valve is moved back to the closed position, causing the gas flow to cease. The pressure quickly returns to an acceptable level, and the indicator light is extinguished. Often, the pressure rises so fast that by the time the user looks to determine if an inadequate gas supply is the problem, the light is no longer lit. This causes the user to believe the problem is something other than the gas supply, and can result in downtime, warranty costs, etc. A similar problem can occur for other temporary losses of pressure, such as when the loss occurs after cutting has been performed for a period of time.
Thus, there is a need for a plasma cutter that senses a marginally inadequate gas pressure, and/or to indicate to the user the gas pressure is inadequate, and to maintain that indication even if the pressure returns to an acceptable level.
According to a first aspect of the invention a plasma cutting power supply provides power to a plasma torch. It includes a gas flow control that regulates the flow of gas to the torch and a power source that provides power to the torch. A controller includes a delay circuit. A gas pressure sensor is connected to the torch and/or the gas flow control, and/or the controller. A user indicator is connected to the delay circuit.
According to a second aspect of the invention a plasma cutter for cutting with a torch includes a gas flow control connected to the torch and a power source connected to the torch. A pressure sensor is connected to the gas flow. A comparison circuit is connected to at least one threshold and the pressure sensor, and provides an output connected to the flow control switch, a power disable circuit, and a user-indicator. The user indicator is also connected to a delay circuit.
The sensor is a dynamic sensor in one alternative.
The delay circuit includes a timing delay circuit, a reset input, and a user-selectable reset input in various alternatives. The reset input is connected to the trigger and/or a cutting parameter feedback circuit in additional alternatives.
The controller further includes circuitry to disable the output in response to the gas pressure sensor in another alternative.
According to a third aspect of the invention a method of plasma cutting with a torch includes providing gas to the torch, regulating the gas flow, and providing cutting power to the torch. The gas pressure is sensed and power and gas flow to the torch are halted if the gas pressure is lower than a threshold. A user indicator is activated and maintained even if the pressure rises when the gas flow is stopped.
The pressure is sensed dynamically in one alternative.
The indication is maintained for a predetermined period of time, until the user resets the indicator, or until current flows in the torch in various alternatives.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.